1. Field of the Invention
This invention involves the preparation of Mg-containing non-Al anionic clays.
2. Description of the Prior Art
Anionic clays have a crystal structure which consists of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic clay wherein OHxe2x88x92 is the predominant anion present.
In hydrotalcite-like anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers contain anions such as NO3xe2x88x92, OH, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92, SO42xe2x88x92, SiO32xe2x88x92, CrO42xe2x88x92, BO32xe2x88x92, MnO4xe2x88x92, HGaO32xe2x88x92, HVO42xe2x88x92, ClO4xe2x88x92, BO32xe2x88x92, pillaring anions such as V10O28xe2x88x926 and MO7O246xe2x88x92, monocarboxalates such as acetate, dicarboxalates such as oxalate, alkyl sulfonates such as laurylsulfonate.
It should be noted that a variety of terms is used to describe the material which is referred to in this patent as an anionic clay. Hydrotalcite-like and layered double hydroxide are interchangeably used by those skilled in the art. In this patent application we refer to the materials as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials.
The most commonly described anionic clays are Mgxe2x80x94Al anionic clays. In the prior art the emphasis is usually on this type of anionic clays, whereas the Mg-containing non-Al anionic clays are only mentioned in passing, even though the chemistry of their preparation and their properties can be very different and unpredictable. Mgxe2x80x94Al anionic clays are suitable for many applications in the absorbent and catalysts field, but Mg-containing non-Al anionic clays have specific applications in these fields.
The preparation of anionic clays has been described in many prior art publications. Two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarized, F. Cavani et al xe2x80x9cHydrotalcite-type anionic clays: Preparation, Properties and Applications,xe2x80x9d Catalysis Todayxe2x80x9d, 11 (1991) Elsevier Science Publishers B. V. Amsterdam. J P Besse and others xe2x80x9cAnionic clays: trends in pillaring chemistry, its synthesis and microporous solidsxe2x80x9d (1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N.Y.
In these reviews basically two types of anionic clay preparation are described. The most conventional method is co-precipitation (in Besse this method is called the salt-base method) of a soluble divalent metal salt and a soluble trivalent metal salt, optionally followed by hydrothermal treatment or aging to increase the crystallite size. The second method is the salt-oxide method in which a divalent metal oxide is reacted at atmospheric pressure with a soluble trivalent metal salt, followed by aging under atmospheric pressure. This method has only been described for the use of ZnO and CuO in combination with soluble trivalent metal salts.
For work on anionic clays, reference is given to the following articles:
Helv. Chim. Acta, 25, 106-137 and 555-569 (1942)
J. Am. Ceram. Soc., 42, no. 3, 121 (1959)
Chemistry Letters (Japan), 843 (1973)
Clays and Clay Minerals, 23, 369 (1975)
Clays and Clay Minerals, 28, 50 (1980)
Clays and Clay Minerals, 34, 507 (1996)
Materials Chemistry and Physics, 14, 569 (1986).
In addition there is an extensive amount of patent literature on the use of anionic clays and processes for their preparation.
European Patent Application 0 536 879 describes a method for introducing pH-dependent anions into the clay. The clay is prepared by the addition of a solution of Al(NO3)3 and Mg(NO3)2 to a basic solution containing borate anions. The product is then filtered, washed repeatedly with water, and dried overnight. Additionally mixtures of Zn/Mg are used.
In U.S. Pat. No. 3,796,792 by Miyata et al. entitled xe2x80x9cComposite Metal Hydroxidesxe2x80x9d a range of materials is prepared into which an extensive range of cations is incorporated, including Sc, La, Th, In, etc. In the examples given solutions of the divalent and trivalent cations are prepared and mixed with base to cause co-precipitation. The resulting products are filtered, washed with water, and dried at 80xc2x0 C. Example 1 refers to Mg and Sb and Example 3 to Mg and Bi. Other examples are given, and in each case soluble salts are used to make solutions prior to precipitation of the anionic clay at high pH.
In U.S. Pat. No. 3,879,523 by Miyata entitled xe2x80x9cComposite Metal Hydroxidesxe2x80x9d also a large number of preparation examples is outlined. The underlying chemistry, however, is again based on the co-precipitation of soluble salts followed by washing and drying. It is important to emphasize that washing is a necessary part of such preparations, because to create a basic environment for co-precipitation of the metal ions a basic solution is needed and this is provided by NaOH/Na2CO3 solutions. Residual sodium, for example, can have a significant deleterious effect on the subsequent performance of the product as a catalyst or oxide support.
In U.S. Pat. No. 3,879,525 (Miyata) very similar procedures are again described.
In U.S. Pat. No. 4,351,814 to Miyata et al. a method for making fibrous hydrotalcites is described. Such materials differ in structure from the normal plate-like morphology. The synthesis again involves soluble salts. For example, an aqueous solution of a mixture of MgCl2 and CaCl2 is prepared and suitably aged. From this a needle-like product Mg2(OH)3Cl.4H2O precipitates. A separate solution of sodium aluminate is then reacted in an autoclave with the solid Mg2(OH)3Cl.4H2O and the product is again filtered, washed with water, and dried.
In U.S. Pat. No. 4,458,026 to Reichle, in which heat-treated anionic clays are described as catalysts for aldol condensation reactions, again use is made of magnesium and aluminum nitrate salt solutions. Such solutions being added to a second solution of NaOH and Na2CO3. After precipitation the slurry is filtered and washed twice with distilled water before drying at 125xc2x0 C.
In U.S. Pat. No. 4,656,156 to Misra the preparation of a novel absorbent based on mixing activated alumina and hydrotalcite is described. The hydrotalcite is made by reacting activated MgO (prepared by activating a magnesium compound such as magnesium carbonate or magnesium hydroxide) with aqueous solutions containing aluminate, carbonate and hydroxyl ions. As an example the solution is made from NaOH, Na2CO3 and Al2O3. In particular, the synthesis involves the use of industrial Bayer liquor as the source of Al. The resulting products are washed and filtered before drying at 105xc2x0 C.
In U.S. Pat. No. 4,904,457 to Misra a method is described for producing hydrotalcites in high yield by reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions.
The methodology is repeated in U.S. Pat. No. 4,656,156.
In U.S. Pat. No. 5,507,980 to Kelkar et at al. a process is described for making novel catalysts, catalyst supports, and absorbers comprising synthetic hydrotalcite-like binders. The synthesis of the typical sheet hydrotalcite involves reacting pseudo-boehmite to which acetic acid has been added to peptize the pseudo-boehmite. This is then mixed with magnesia. More importantly, the patent summary states clearly that the invention uses mono carboxylic organic acids such as formic, propionic and isobutyric acid. In this patent the conventional approaches to preparing hydrotalcites are presented.
In U.S. Pat. No. 6,539,861 a process is disclosed for preparing a catalysts for synthesis gas production based on hydrotalcites. The method of preparation is again based, on the co-precipitation of soluble salts by mixing with base, for example, by the addition of a solution of RhCl3, Mg(NO3)2and Al(NO3)3 to a solution of Na2CO3 and NaOH.
Also in U.S. Pat. No. 5,399,537 to Bhattacharyya in the preparation of nickel-containing catalysts based on hydrotalcite use is made of the co-precipitation of soluble magnesium and aluminum salts.
In U.S. Pat. No. 5,591,418 to Bhattacharyya a catalyst for removing sulfur oxides or nitrogen oxides from a gaseous mixture is made by calcining an anionic clay, said anionic clay having been prepared by co-precipitation of a solution of Mg(NO3)2, Al(NO3)3 and Ce(NO3)3. The product again is filtered and repeatedly washed with de-ionized water.
In U.S. Pat. No. 5,114,898/WO 9110505 Pinnavaia et al. describe layered double hydroxide sorbents for the removal of sulfur oxide(s) from flue gases, which layered double hydroxide is prepared by reacting a solution of Al and Mg nitrates or chlorides with a solution of NAOH and Na2CO3. In U.S. Pat. No. 5,079,203 /WO 9118670 layered double hydroxides intercalated with polyoxo anions are described, with the parent clay being made by co-precipitation techniques.
In U.S. Pat. No. 5,578,286 in the name of Alcoa a process for the preparation of meixnerite is described. Said meixnerite may be contacted with a dicarboxalate or polycarboxalate anion to form a hydrotalcite-like material.
In U.S. Pat. Nos. 4,946,581 and 4,952,382 to van Broekhoven co-precipitation of soluble salts such as Mg(NO3)2 and Al(NO3)3 with, and without the incorporation of rare earth salts was used for the preparation of anionic clays as catalyst components and additives. A variety of anions and di- and tri-valent cations are described.
As indicated in the description of the prior art given-above, there are many applications of anionic clays. These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular van Broekhoven has described their use in SOx abatement chemistry.
Martin et al (U.S. Pat. No. 5,728,364) involves the formation of a hydrotalcite clay by means of a complex multi-step process with at least two steps involving (a) a reaction to give an intermediate layered hydroxide and (b) the addition of an anion source and further reaction. The teachings of this patent emphasize that an intermediate material i.e. an xe2x80x9cmeixnerite-like intermediatexe2x80x9d (column 7 line 14), must be obtained prior to the second step where the layered double hydroxide +H2O is produced and that this intermediate be produced in a reaction free of ions of carboxylic acid and carboxalate.
Stamires et al (U.S. Pat. No. 6,171,991) teaches a process for making an anionic clay-containing composition that includes the use of an aluminum source. The resulting composition comprises an Mgxe2x80x94Al anionic clay and an Mgxe2x80x94Al anionic clay composited with alumina.
Because of this wide variety of large-scale commercial applications for these materials, new processes utilizing alternative inexpensive raw materials are needed to provide a more cost-effective and environmentally compatible processes for making anionic clays. In particular, from the prior art described above one can conclude that the preparation process can be improved in the following ways: the use of cheaper sources of reactants, processes for easier handling of the reactants and with fewer steps, so that there is no need for washing or filtration, eliminating the filtration problems associated with these fine-particled materials (which are very difficult to filter and wash), the avoidance of alkali metals (which can be particularly disadvantageous for certain catalytic applications): Further, in drying or calcining the anionic clay prepared by prior art processes gaseous emissions of nitrogen oxides, halogens, sulfur oxides, etc. are encountered which cause environmental pollution problems.
In its broadest embodiment, our invention comprises a process for the preparation of a magnesium-containing non-Al anionic clay. The process comprises a one-step process wherein an aqueous suspension comprising a trivalent metal source and at least a magnesium containing source as a divalent metal source is provided and reacted to obtain a magnesium-containing non-Al anionic clay. The divalent or trivalent metal source or both comprises a hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate anions or mixtures thereof. There may be a source of hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate anions or mixtures thereof, other than anions associated with the divalent or trivalent metal. The anionic clay has interlayers containing anions comprising hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate and/or formate anions or mixtures thereof (hydroxyl anions tend to be displaced by the other anions).
Other embodiments of our invention encompass details about compositions, manufacturing steps, etc., all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.
Our invention includes processes for producing Mg-containing non-Al anionic clays using relatively inexpensive starting materials in a simple process which involves reacting mixtures with or without stirring in water, optionally under hydrothermal conditions. Such processes can be operated in standard laboratory/industrial equipment. More specifically, there is no need for washing or filtering, and a wide range of ratios of Mg(II)/M(III) in the reaction product is possible.
This invention involves the use of a trivalent metal source other than aluminum and a divalent metal source in aqueous suspensions, which are reacted, optionally under hydrothermal conditions and the reaction mixture results in the direct formation of a Mg-containing non-Al anionic clay. The powder X-ray diffraction pattern (PXRD) suggests that the product is comparable to anionic clays made by other standard methods. The physical and chemical properties of the product are also comparable to those anionic clays made by the other conventional methods. The overall process of this invention is very flexible, enabling a wide variety of anionic clay compositions and anionic clay-like materials involving for example hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate and mixtures thereof to be prepared in an economically and environmental-friendly manner. The process may be carried out in a one-step process either in batch or in continuous mode.
This invention involves the preparation of Mg-containing non-Al anionic clays. In particular it describes a process for the preparation of an anionic clay wherein a suspension comprising a trivalent metal source and a Mg source is provided and reacted thermally or hydrothermally to obtain a Mg-containing non-Al anionic clay having interlayers that contain anions including hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate anions or mixtures thereof.
It was found that Mg-containing non-Al anionic clays are directly obtained from the reaction according to the invention. This is in contrast to the co-precipitation method wherein soluble salts are first precipitated, filtered, washed to remove unwanted ions and then aged either hydrothermally or not. With the process according to the invention the presence of undesireable ions in the product can be avoided, as will be explained below. If for the trivalent metal source a compound is chosen with harmless ions such as hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate, or mixtures thereof, washing and filtration of the reaction product can be avoided altogether. In fact, it was found that the reaction also takes place when using hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate anions, or mixtures thereof for both the divalent metal source and the trivalent metal source, in which case also no washing and filtration has to take place.
Alternatively, the divalent and/or trivalent metal sources may be oxides, hydroxides and mixtures thereof, in conjunction with the hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, and/or formate anions or mixtures thereof, the latter of which will serve to preferentially displace hydroxy anions.
The process of the present invention aims at a single step process and in fact requires that hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate anions or mixtures thereof be provided to the process. Thus, completely opposite Martin et al, as described above, the process of the present invention does not require a meixnerite type intermediate phase containing hydroxide anions, but requires the presence of hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate anions, or mixtures thereof in its single step. The instant process is thus significantly simpler and more cost effective than the two step process described in Martin et al.
Martin et al also teaches an additional reaction of the intermediate of its process with an acid to first form an intermediate and then exchange the intermediate with an acid. It is not necessary in the process of the present invention to add an acid, i.e. if an acid (or base) is added it is as part of the one-step process and acid (or base) is added to control pH and not for the purpose of providing anions.
The above discussed Stamires et al, is diametrically opposite the present invention, in that it requires aluminum in the clay composition.
Since the process disclosed in this patent does not require washing of the product or filtering, there is no filtrate waste or gaseous emissions (e.g. from acid decomposition), making the process particularly environmental-friendly and more suited to the environmental constraints which are increasingly imposed on commercial operations. The product can be spray dried directly to form microspheres or can be extruded, pelletized or beaded to form shaped bodies.
Anionic clays prepared by this method exhibit the well known properties and characteristics (e.g. chemical analysis, powder X-ray diffraction pattern, FTIR, thermal decomposition characteristics, surface area, pore volume, and pore size distribution) usually associated with anionic clays prepared by the customary and previously disclosed methods.
The anionic clay according to the invention has a layered structure corresponding to the general formula
[Mg(II)m2+M(III)n3+(OH)2m+2n.]Xn/zzxe2x88x92.bH2O
Wherein m and n have a value such that m/n=1 to 10, preferably 1 to 6, and b has a value in the range of from 0 to 10, generally a value of 2 to 6 and often a value of about 4. X comprises hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate or mixtures thereof. It is more preferred that m/n should have a value of 2 to 4, more particularly a value close to 3.
Suitable trivalent metal sources are compounds containing Mn3+, Fe3+, Co3+, Ni3+, Cr3+, Ga3+, B3+, trivalent rare earth metal cations such as La3+ and Ce3+, Ga3+, or mixtures of said compounds. Preferably oxides, hydroxides, carbonates, or hydroxycarbonates, or mixtures thereof, of these metals are used, but also bicarbonates, acetates, hydroxyacetates, oxalates, nitrates, hydroxylates, oxides and/or formates, or mixtures thereof can be used.
Suitable Mg sources are MgO, Mg(OH)2, and magnesium salts such as magnesium hydroxycarbonate, magnesium acetate, magnesium hydroxyacetate, magnesium carbonate, and magnesium bicarbonate. In addition to the Mg source other divalent metal source may be added such as compounds containing Mg2+, Ca2+, Zn2+, Mn2+, Co2+, Ni2+, Fe2+, Sr2+, Ba2+, Cu2+, Sn2+, Cr2+, Cd2+, V2+, and mixtures of said compounds. Preferably oxides, hydroxides, hydroxycarbonates, carbonates and bicarbonates of these metals are used, but also nitrates chlorides, sulfates, phosphates, acetates, hydroxyacetates, formates, oxalates, borates, tungstates, molybdates, vanadates or mixtures thereof can be used.
The divalent and trivalent metal sources may be associated with any of the anions as described above, so long as the interlayers contain anions that include a hydroxycarbonate, carbonate, bicarbonate, acetate, hydroxyacetate, oxalate, nitrate, oxide and/or formate or mixtures thereof.
As mentioned above, the reaction is conducted under thermal or hydrothermal conditions. Within the context of this description hydrothermal means in the presence of water at at a temperature above 100xc2x0 C. at increased pressure. Thermal means at a temperature between ambient and 100xc2x0 C. in air or other atmosphere. Preferably the reaction takes place in water in an autoclave at a temperature above 100xc2x0 C., i.e. under autogeneous pressure.
Alternative to providing carbonate anions as discussed above, the reaction can be conducted in the presence of CO2. The CO2 may be derived from the CO2 normally present in air or it may be added to the reaction separately by carbonate present in the water in the reaction mixture or by adding reactants comprising compounds bearing carbonate or bicarbonate anions, for instance, by using a carbonate divalent or trivalent metal source. Hydroxycarbonate, acetate, hydroxyacetate, oxalate, nitrate, hydroxyl, oxide and/or formate anions may also be added to the reaction separately by being present in the reaction mixture, or be provided by one or more of the reactants.
Said aqueous suspension may be obtained by either combining slurries of the starting materials or adding divalent metal source to a slurry of trivalent metal source or vice versa. There is no need to wash or filter the product, as unwanted ions (e.g. sodium, chloride, sulfate)which are frequently encountered when using other preparation methods, are absent in the product. If desired a preformed anionic clay may be added as a seed to the reaction mixture. Said preformed clay may be recycled anionic clay from the reaction mixture or anionic clay made separately by the process according to the invention or any other process.
Because of its simplicity, this process can be carried out in a continuous mode by mixing of a first slurry comprising a trivalent metal source and a second slurry comprising magnesium source passing the mixed slurry through a reactor vessel which can operate under hydrothermal conditions. Said first and/or second slurry may be subjected to a treatment prior to mixing the slurries. Said pre-treatment may involve treatment with acid, base treatment, thermal and/or hydrothermal treatment, all optionally in the presence of seeds or combinations thereof.
The non-soluble ingredients of the aqueous suspension comprising the reaction mixture may be milled or homogenized in an aqueous medium, either before being added to the reaction mixture, or while part of the reaction mixture.
In this specification the tern xe2x80x98millingxe2x80x99 is defined as any method that results in reduction of the particle size. Such a particle size reduction can at the same time result in the formation of reactive surfaces and/or heating of the particles. Instruments that can be used for milling include ball mills, high-shear mixers, colloid mixers, and electrical transducers that can introduce ultrasound waves into a slurry. Low-shear mixing, i.e. stirring that is performed essentially to keep the ingredients in suspension, is not regarded as xe2x80x98millingxe2x80x99.
Homogenization may be carried out by mechanical means such as high shear mixing or by the use of ultra-sound.
As mentioned-above, if desired inorganic or organic acids and bases, for example for control of the pH, may be added to the slurry before or during reaction or to the individual reactants before combining them in the slurry. The acid and bases of choice are formic acid, acetic acid, nitric acid, oxalic acid, ammonium hydrogen phosphate, ammonium phosphate, phosphoric acid, ammonium hydroxide, ammonium carbonate and ammonium bicarbonate, because these types of acids and bases do not introduce unwanted ions in the reaction mixture.
The most preferred combinations of a divalent metal source and a trivalent metal source are Gaxe2x80x94Mg, Fexe2x80x94Mg, Crxe2x80x94Mg, Laxe2x80x94Mg, Cexe2x80x94Mg, Fexe2x80x94Zn/Mg, Laxe2x80x94Zn/Mg, Cexe2x80x94Zn/Mg, Gaxe2x80x94Zn/Mg, Gaxe2x80x94Fe/Mg, Crxe2x80x94Zn/Mg, Crxe2x80x94Cu/Mg, Crxe2x80x94Mn/Mg and Fexe2x80x94Co/Mg because these combinations result in Mg-containing non-Al anionic clays with specific applications in the catalyst field.
If desired, the anionic clay prepared by the process according to the invention may be subjected to ion exchange or pillaring. Upon ion exchange the interlayer charge-balancing anions are replaced with other anions. Said other anions are the ones commonly present in anionic clays and include pillaring anions such as V10O28xe2x88x926, Mo7O246xe2x88x92 and other anions known in the art for use in ionic clays. Said ion exchange can be conducted before drying or after the anionic clay formed in the slurry.
The anionic clay prepared by the process according to the invention may be aged for a period of from about 5 minutes to about 5 days.
The products of this invention may be calcined to form solid solutions which may contain spinels. The calcining may be carried out at a temperature from about 300xc2x0 C. to about 1,000xc2x0 C. to form a solid solution.
The solid solution may be hydrated by treatment, in an aqueous slurry that may contain additives comprising soluble compounds, thermally at temperatures not more than about 100xc2x0 C., or hydrothermally at a temperature over about 100xc2x0 C. at autogeneous pressure.
The calcined materials may be rehydrated by treatment in aqueous slurries that may contain additives comprising soluble compounds. The products may be shaped before or after aging by conventional means, such as spray drying or extrusion.
The process of the invention provides wide flexibility in preparing products with a wide range of M(II):M(III) ratios. The M(II):M(III) ratio can vary from 0.1 to 10 preferably from 1 to 6, more preferred from 2 to 4, and especially preferred to close to 3.
For some applications it is desirable to have additive compounds containing one or more of metals and non-metals, such as rare earth metals, noble metals, Si, P, B, Ti, Zr, group VI, group VIII, alkaline earth (for instance Ca and Ba) and/or transition metals (for example Mn, Fe, Ti, Zr, Cu, Ni, Zn, Mo, Sn, V, Co and W), present. Said metals can easily be deposited on the anionic clay. They can also be added either to the divalent metal source or the trivalent metal source or to the slurry during preparation of the anionic clay.
The present invention is illustrated by the following examples that are not to be considered limiting by any means. It is understood that anions associated with the reactants will appear in the interlayers.